Effect of muscle stretching on tension development and mechanical threshold during contractures

Neuromuscular transmission to identified primary and secondary myotubes: a reevaluation of polyneuronal innervation patterns in rat embryos

The early morphogenesis of rat skeletal muscle is a biphasic process involving two sequentially generated populations of myotubes. A small population of primary myotubes appears early and is followed by a much larger population of secondary myotubes which appear progressively over a number of days. All previously published electrophysiological studies of developing muscle have failed to appreciate the relevance of biphasic myotube production. Here we reevaluate the status of early motor innervation, taking into account the wide range of sizes and levels of maturity within the two myotube populations. Evoked end-plate potentials (EPPs) were recorded from fibers of E17-20 rat sternocostalis muscles. Impaled fibers were then marked by ejection of HRP from the recording pipet, enabling ultrastructural identification of fibers from which recordings had been made. The average number of synaptic inputs per fiber increased to a peak at E19, and the rate of rise of the EPPs increased with age. The majority of impaled fibers (76%) were subsequently found to be primary myotubes, even at ages when secondary myotubes formed the majority of fibers in the muscle. Electrophysiological studies during early stages of secondary myotube development therefore sample largely from the more mature primary fibers and probably give the wrong impression of the extent and degree of polyneuronal innervation and of synaptic rearrangement within the muscle. In addition, the results show that very young secondary myotubes are distinguished by EPPs of longer latency, slower rate of rise, and smaller size than those of other types of myotubes. These results suggest that young secondary myotubes are predominantly activated by EPPs which originate in adjoining primary myotubes and propagate electronically to the secondary myotube. We propose a new model of early synaptic rearrangement which accommodates the biphasic nature of muscle development. We also suggest that secondary myotubes do not require direct neural input for the initiation of their development.

The effect of stretch on sarcoplasmic free calcium of frog skeletal muscle at rest

The concentration of free calcium in the sarcoplasm of a resting frog skeletal muscle is increased by stretch. The magnitude of the rise in free calcium increased with the degree of stretch and the ambient temperature and it was enhanced by caffeine. This phenomenon might play a role in the twitch potentiation and enhanced metabolic rate evoked by stretch.

Effects of catecholamines and cyclic amp on excitation--contraction coupling in isolated skeletal muscle fibres of the frog

1. In skeletal muscle the presence of a positive inotropic effect induced by adrenaline has been a matter of controversy. If it exists, it could be due to catecholamines acting on the actomyosin system, on the sarcoplasmic reticulum (SR) Ca2+ pump or on the release or influx of Ca2+. We investigated these possibilities by using intact, split and skinned skeletal muscle fibres. We also investigated whether adrenaline acts directly or through cyclic AMP. 2. Catecholamines produced an increase in twitch tension and in maximum rates of tension development and tension decay. The inotropic effect took 3 min to appear and 8 min to reach its maximum level. With tetanic stimulations the extra force appeared only at the beginning of the tetanus while approaching the same maximum level, and tended to disappear faster, the higher the frequency of stimulation. At 4 shocks/sec the peak twitch tension with catecholamines decreased during the first seven to ten twitches and became steady afterwards at a level that was still greater than the control. 3. Resting and action potentials showed no important changes in the presence of adrenaline that could explain the inotropic effect. 4. In split fibres the force produced with the release of Ca2+ from the SR by caffeine was 60-100% larger when cyclic AMP was added to the previous loading solution. In skinned fibres adrenaline given directly to the interior of the cell produced no changes in contraction--relaxation cycles induced by fixed amounts of Ca2+ applied with a pipette. 5. These results strongly suggest that catecholamines through cyclic AMP stimulate the SR Ca2+ pump, increasing thereby the concentration of Ca2+ within the SR. This extra Ca2+ when released during subsequent activation may produce the increase in twitch tension.

Graded activation of myofibrils and the effect of diameter on tension development during contractures in isolated skeletal muscle fibres

If the space constant of the T-system (lambdaT) its not large in comparison with the radius (a) of a muscle fibre, different levels of depolarization should activate different proportions of the cross-section. This possibility was tested in isolated muscle fibres with isotonic and isometric K contractures. 2. During isonic contractures with more than 40 mM-K, wavy myofibrils appeared in the centre of the fibre. The sarcomere spacings (s) of the wavy myofibrils, measured parallel to the long axis of the myofibrils, were 1-9-1-95 mum. However, the superficial myofibrils could shorten to or below s=1-5 mum without becoming wavy. 3. In the same muscle fibre where myofibrils became wavy during K contractures, no waviness appeared during repetitive electric stimulation in normal Ringer (50 shocks/sec, 12 degrees C), although s decreased below 1-5 mum. Wavy myofibrils were interpreted as not activated. 4. With isometric contractures it was found that the amount of depolarization needed to obtain maximal tension was smaller for fibres of shorter radius. The degree of depolarization for producing maximal tension is related to a by 6 mV/10mum. 5. These results strongly suggest that in K contractures lambdaT is not large in comparison with a.